Low-pressure arc-discharge tube supplied with direct current



D. KOLKMAN ET AL 2,748,308

LOW-PRESSURE ARC-DISCHARGE TUBE SUPPLIED WITH DIRECT CURRENT May 29,1956 Filed June 12, 1955 INVENTORS DIRK KOLKMAN ADOLF JAN DE VISSERAGENT United States Patent LOW-PRESSURE ARC-DISCHARGE TUBE SUP- PLIEDWITH DIRECT CURRENT Application June 12, B53, Serial No. 361,342

6 Claims. (Cl. 313-185) This invention relates to low-pressurearc-discharge tubes supplied with direct current, more particularlyfluorescent lamps, which comprise a gas-filling having a pressure of 2to 5 mm. of mercury, two activated main electrodes located at the endsof the tube, and two strip-shaped auxiliary electrodes consisting ofresistance material and provided on the inner wall of the tube. Saidauxiliary electrodes extend substantially throughout the length of thetube, are each time conductively connected to one of the main electrodeswithin the discharge space and insulated from the other main electrode.

Upon connecting such tube through a series resistor to a suitable directvoltage a glow discharge is struck between the main electrode connectedas a cathode and the auxiliary electrode insulated from it and connectedas an anode, the glow discharge propagating along the auxiliaryelectrode and finally finding its way to the main electrode connected asan anode. The glow discharge changes into an arc discharge upon theactivated cathode attaining its emission temperature.

The tubes comprise two main electrodes and two auxiliary electrodes sothat, independently of the polarity of the source of direct current,always either of the main electrodes acts as a cathode and either of theauxiliary electrodes acts as an anode of the glow discharge.

This further permits of inversing the polarity of the tube duringoperation or during idle hours, thus preventing the metal vapour, forexample in fluorescent lamps, from concentrating near the cathode.

Inversion of polarity is mainly effected in the idle hours by means of asuitable main switch in such manner that the tube is each timedifferently polarised in the successive operating conditions usuallylasting at most several hours.

It has now been found, that ignition difficulties may arise inconnection with the inversion of polarity. If the tube has been operatedonly for a few seconds, followed by several idle hours, and the tube issubsequently switched on in the condition of inversed polarity, it maynamely happen that the tube fails to ignite.

The present invention has inter alia for its purpose to mitigate thisinconvenience.

According to the invention only one end of each main electrode isconnected to an external contact of the tube, the other end beingprovided with an auxiliary conductor which extends in the direction ofthe auxiliary electrode insulated from the said main electrode.

During operation the auxiliary conductor of the main electrode, whichelectrode is connected as an anode, takes a part of the arc-dischargecurrent and passes this part via the non-negligible resistance of themain electrode to the external contact of the tube end containing themain electrode acting as an anode, thus heating the activated part ofsaid main electrode. It has unexpectedly been found that this permits ofobviating the aforesaid ignition difiiculties.

The auxiliary conductor moreover fulfills another im- 2,748,308 PatentedMay 29, 1956 'ice 2 portant function. Since it extends in the directionof the auxiliary electrode, which is insulated from the main electrodeto which the auxiliary conductor is connected, the auxiliary conductor,if connected as a cathode, promotes the striking of the glow discharge.

The lowest voltage at which the glow discharge strikes is attained ifthe minimum distance in millimeters between the auxiliary conductor andthe auxiliary electrode insulated from it is between l2/p and 40/ p,preferably between 15/ p and 30/ p, where p represents the pressure ofthe gasfilling in millimeters mercury at room temperature.

The auxiliary conductor, viewed from the discharge space, isadvantageously situated behind the activated part of the main electrodeconnected to it. Thus it is ensured that blackening of the tube wall,due to the presence of the auxiliary conductor, occurs where theemission of light or other radiations must not be impeded, the auxiliaryconductor yet receiving a part of the discharge current sufficient toheat the main electrode adequately.

Activated electrodes are often surroundedby an insulated screen tocollect the particles disintegrated from them. When using such screensin a tube according to the invention it has been found imperative thatthe auxiliary conductor should terminate withinthescreen near the mainelectrode end connected to the contact, lest the passing over of theglow discharge into the arc discharge be retarded or even prevented.

The auxiliary conductor preferably consists of nickel or iron. If madeof tungsten, for example, it has been found that it fulfils itsfunctions worse.

In order that the invention may be readily carried into effect, it willnow be described with reference to the accompanying drawing whichrepresents one example thereof and in which Fig. l is a perspective viewof a discharge tube according to the invention, Fig. 2 being a plan viewof the lower part of this tube.

With reference to the drawing, the reference numeral 1 denotes thecylindrical, glass discharge vessel approximately cm. long andapproximately of 35 mm. inner diameter of the tube. The discharge vesselis filled with argon gas under a pressure of approximately 3 mm. at roomtemperature and contains a small quantity of mercury (approximately 30mg). The inner wall of the discharge vessel is provided with a layer(not shown) capable of converting ultraviolet rays of the lowpressuremercury-vapour discharge into radiations of greater wavelength duringoperation.

The discharge tube comprises two coiled-coil main electrodes 2 and 3respectively which are activated by the conventional alkaline earthcompounds and the ends of which are secured to supporting wires 4, 5 and6, 7 respectively. The supporting wires are carried by the usual pinchconstructions closing the discharge vessel in a vacuum-tight manner. Thesupporting wires 5 and 7 respectively are conductively connected tocontacts 8 and 9'respectively fitted to the-outer wall of tube feet 10and 11 respectivelyat the ends of the tube.

The supporting wires 4 and 6 respectively, not connected to a contact,are linked to band-shaped auxiliary electrodes 16 and 17 respectivelythrough metal wires 12 and 13 respectively and quadrantal metal strips 1and 15 respectively. Said auxiliary electrodes consist of a mixture ofgraphite and glaze, have a resistance of approximately 25 ohms per cm.length of the band and extend parallel with the axis of the dischargevessel on its inner wall over a width of approximately 3 mm. andsubstantially throughout the length of the discharge vessel.

Thus, the auxiliary electrode 16 is conductively connected to the mainelectrode 2 and insulated from the main electrode 3, whereas theauxiliary electrode 17 is conductively connected to the main electrode 3and insulated from the main electrode 2.

Welded to the metal wire 12 of the main electrode 2 is an auxiliaryconductor consisting of nickel wire approximately 2 mm. thick, whichcomprises three horizontal parts 18, 19, 20, and a vertical part 21. Thehorizontal parts are U-shaped, their plane extending substantially atright angles to the plane through the supporting Wires 4 and 5 of themain electrode 2. The vertical part extends substantially parallel withthe supporting wire 4 and may be situated in the plane through thesupporting wires 4 and 5.

Secured to the metal Wire 13 of the main electrode 3 is a correspondingauxiliary conductor also comprising three horizontal parts 22, 23 and 24and a vertical part 25 (see also Fig. 2).

The part 19 of the auxiliary conductor of the main electrode 2 and thepart 23 of the auxiliary conductor of the main electrode 3 are nearestto the auxiliary electrodes 16 and 17 respectively insulated from themain electrode concerned. The minimum spacing between the auxiliaryconductor and the auxiliary electrode (the spacing between the part 23and the auxiliary electrode 16 in Fig. 2) is approximately 8 mm., hencethis spacing corresponds to approximately 24/p, where p represents thepressure of approximately 3 mm. of the gasfilling.

Each main electrode is surrounded by the usual, annular, metal,disintegration screen, shown only in Fig. 2 as the screen 26 of the mainelectrode 3. This screen comprises a tag 27 which is soldered to thesupporting wire 28 (also shown in Fig. 1). Fig. 1 shows a similarsupporting wire 29 which is sealed into the pinch of the main electrode2 and serves for the connection to the tag of the screen for the mainelectrode 2.

The tube shown is a 40 w. fluorescent lamp which, upon connection via anincandescent lamp of 110 v., 40 w. to a direct voltage of approximately220 v. has a tube voltage of approximately 110 v. and takes anarc-discharge current of approximately 360 ma.

Measurements have shown that the auxiliary conductor connected as ananode takes approximately 150 ma. of the arc-discharge current duringoperation and passes it through the coils of the associated mainelectrode to the positive terminal of the source of current. Due to theresulting heating of the main electrode the ignition difiicultiesreferred to are obviated.

By the said spacing of approximately 8 mm. of the auxiliary conductorfrom the auxiliary electrode it is achieved that the tube strikes at adirect voltage as low as 180 volts and attains the arc-dischargecondition, so that the tube is substantially independent of voltagedrops that may occur in direct current mains of 220 volts nominal.

When using insulated disintegration screens it has found to be necessaryto provide the auxiliary conductors with the vertical parts 21 and 25respectively and to locate these parts within the screen near thesupporting wires 5 and 7 respectively connected to the external contacts8 and 9 respectively, since otherwise the arc-discharge condition is notreached or only very slowly, which detracts from the life of the tube.

The horizontal parts of the auxiliary conductors not surrounded by thedisintegration screen are nearer to the tube end concerned than theassociated main electrode,

thus achieving that blackening of the discharge vessel from theauxiliary conductors can only occur beyond the discharge path. The factthat the auxiliary conductors point towards an auxiliary electrodemoreover involves that any blackening from the auxiliary conductormainly occurs close to said auxiliary electrode. However, the dischargetube is preferably positioned so as to remove the auxiliary electrodesfrom sight, hence any possible blackening is harmless.

What is claimed is:

1. A low-pressure arc-discharge tube for direct current operation,comprising an elongated envelope having a gas-filling at a pressure ofabout 2 to 5 mm. of Hg., a pair of activated main electrodes each havingtwo terminals and each being located at one end of said envelope andboth defining a discharge space therebetween, a pair of strip-shapedauxiliary electrodes each constituted of resistance material, each ofsaid auxiliary electrodes being mounted on the inner wall of theenvelope and extending substantially throughout the length of the tube,each of said auxiliary electrodes being electrically connected to one ofsaid main electrodes within the discharge space and being insulated fromthe other main electrode, only one external contact member mounted ateach end of the envelope, only one terminal of the main electrodeadjacent each contact member being electrically connected thereto, andan auxiliary conductor electrically connected to the other terminal ofthe main electrode and extending in the direction of but spaced from theauxiliary electrode insulated from said main electrode.

2. A discharge tube as claimed in claim 1, in which the minimum spacingin millimeters of the auxiliary conductor from the auxiliary electrodeinsulated from it is between 12/p and 40/p, p representing the pressureof the gas-filling in mm. mercury at room temperature.

3. A discharge tube as claimed in claim 2, in which the auxiliaryconductor, viewed from the discharge space, is situated behind theactivated part of the main electrode to which it is connected.

4. A discharge tube as claimed in claim 1 in which a main electrode issurrounded by an insulated screen, and the auxiliary conductorterminates within the screen near the main electrode terminal connectedto the contact member. I

5. A discharge tube as claimed in claim 4 in which the auxiliaryconductor consists of a metal selected from the group consisting ofnickel and iron.

6. A low-pressure arc-discharge tube as claimed in claim 1 wherein theauxiliary conductor comprises a U- shaped wire extending transverse tothe envelope axis and having one end connected to said other terminal ofsaid main electrode and the other end terminating in a short axialportion adjacent said one terminal of said main electrode, the plane ofsaid U-shaped wire lying behind the activated part of said mainelectrode.

References Cited in the file of this patent UNITED STATES PATENTS1,961,618 Machlett June 5, 1934 2,042,147 Fairbrother May 26, 19362,094,647 Freitag et al Oct. 5, 1937 2,291,965 Iancke et a1. Aug. 4,1942 2,491,854 Force Dec. 20, 1949

